The detection of loose rock has generally been done up until now by hitting the rock with a scaling bar and listening to the resulting sounds. A "drummy" sound was an indication of loose rock and a "ringing" sound an indication of solid rock. However, the above method, although reasonably effective, proved to be highly subjective and attempts have been made to design an instrument which would be more objective. One such instrument is disclosed in U.S. Pat. No. 4,598,588 which was issued July 8, 1986 and in an article entitled "Bureau (of Mines) develops device to detect unstable roof rock" in Mining Engineering March 1986, page 158. The instrument consists of an accelerometer which is held against the rock using a spring mounted pole, and an analysing device which is held by the miner and is connected to the accelerometer by a coaxial cable. The accelerometer detects sound waves which are generated by lightly tapping the adjacent rock with a striker bar. The sound waves are received and filtered so that the energy levels of two bands -- approximately 500 to 1000 Hz and 3000 to 3500 Hz -- can be compared. Loose rock, relative to solid material, shows a higher energy level in the low frequency band. This is indicated by a higher number on a digital readout device. Fairly good results were obtained in identifying loose rock conditions using this instrument. However, underground testing indicated a certain amount of readout variations due to factors such as degree of contact of the accelerometer with the rock surface, the type of striker bar, the geology etc. In addition, there are several variables which affect the relative energy levels of sound waves as received by the accelerometer. For example, the dampening effect of some rock types could disguise the presence of loose rock by reducing the energy of certain frequencies and diminishing the wave energy relationship. Thus, while the primary indication of "loose/not loose" is valid under most circumstances, the specific numbers that appear on the digital readout of the instrument do not readily provide an indication of the mass of the loose rock or of the degree of looseness. The device is also limited to differentiate loose from solid if the loose is larger than 4m.times.0.5m.times.0.5m.
It is, therefore, the object of the present invention to provide a more reliable loose rock detector and also to provide a detector which can assess mass and degree of looseness of the rock.